Motion detection system

ABSTRACT

An intrusion alarm system for providing an alarm indication in response to the detection of an intruder within a protected area includes Doppler radar apparatus having a transmitter for radiating energy into the protected area, a receiver for receiving Doppler signals provided whenever radiated energy is reflected off a moving body within the protected area and a signal detecting and filter circuit having a lever detector circuit and a low pass digital filter circuit responsive to Doppler signals in excess of a predetermined amplitude and frequency to provide a logic level output indicative of the detection of an intruder within the protected area. In addition to the low pass digital filter circuit, there are described a high pass digital filter circuit and a digital band pass filter circuit.

McLean et al.

[ MOTION DETECTION SYSTEM Primary ExaminerMa nard R. Wilbur v y [75]Inventors: Michael B. McLean, Whitefish Bay; Assistant Exammer (,]regryAllen Rasmussen Milwaukee both Attorney, Agent, or Fzrm.lohnson,Dlenner, Emrich, of Wm Verbeck and Wagner [73] Assignee: 11233121gzrwieiCompany, [57] ABSTRACT An intrusion alarm system for providing analarm indi- [22] med: 1972 cation in response to the detection of anintruder [21] Appl. No.: 220,943 within a protected area includesDoppler radar apparatus having a transmitter for radiating energy intothe protected area, a receiver for receiving Doppler sig- S 343/5 307/nals provided whenever radiated energy is reflected I a s I 1 s s s u ee I n s I n I I n 1 l s u -:'--sa [58] new of Search 343/5 307/233 234nal detecting and filter circuit having a lever detector circuit and alow pass digital filter circuit responsive to [56] References C'tedDoppler signals in excess of a predetermined ampli- UNITED STATESPATENTS tude and frequency to provide a logic level output in- 3,242,4863/1966 Corbell 343/5 PD dicative of the detection of an intruder Withinthe pro- 3.706,96l 12/1972 Sugiura..... 43/5 PD tected area. in additionto the low pass digital filter 5/ 1971 307/233 X circuit, there aredescribed a high pass digital filter g circuit and a digital band passfilter circuit. ven en e a 14 Claims, 14 Drawing Figures DELAY DJ MONOSTABLE T0 ALARM 49 4 r 4 6 CKT 28 DEL/l Y B 42 4 3 44 ADJ --MON0.STABLEL. Q 0 40 PATENTEDAPR 9 m4 3.803.599

SHEET 1 OF 4 22 2/ FIG.

TRAN E SMITTER 27) A SIGNAL DETECTOR AMP RECEIVER 2 ALARM CIRCUIT DELAYE W 'MON0$TABLE 70 ALARM 2 49 ,4/ 46 CKT. 2a

DELAY ADJ '-M0N0$TABL 42 c 43 44 FIG. 2A

OUTPUT LEVEL v F RE OUENC Y APR 9 m4 3 8 O3; 5 9 9 sum 2 or 4 FIG. 4

165 F l 6. 5A fli OUTPUT I l W LEVEL I 01 DETECTOR maouzucr BACKGROUNDOF THE INVENTION 1. Field of the Invention This invention relates tomotion detection, particularly as applied to intrusion alarm systems,and more specifically to a Doppler radar motion detection system fordetecting the frequency and amplitude of Doppler signals resulting fromthe movement of a body within a protected area.

2. Description of the Prior Art Intrusion detection systems previouslyproposed having DOppler radar motion detection apparatus using eitherelectromagnetic or acoustic radiation to produce a Doppler shift havebeen characterized by a number of shortcomings, the most prevelant ofwhich is the undesired detection of noise.

Some type of noise motion will generally be present in any area which isto be protected by a Doppler radar motion detection system. Noise motionis the unwanted yet uncontrollable movement with the protected areawhich may be caused, for example, by wall vibrations or by machinery,fans, fluorescent lights, etc., operating within the protected area.Each of these objects has a movable part, and accordingly, when energyof a predetermined frequency is radiated into the protected area, themovements of these objects will cause the generation of DOppler signals.The Doppler signals provided as the result of such uncontrollablemovement within the protected area appear as ambient noise in thesecured area.

Certain prior art motion detection systems have included thresholddetecting means which is responsive to Doppler signals in excess of apredetermined amplitude to enable an alarm generator for indicating anintrusion of a secured area. However, because of the presence ofuncontrollable ambient noise, the setting of the level detector and thusthe overall sensitivity of the motion detection system had to be reducedto a level whereat ambient noise would not be detected. The alarmthreshold had to be set well above the average noise level to avoidfalse alarms. Even so, short duration, high amplitude pusles could stilloccur causing false alarms to be generated.

To eliminate such problems, certain other prior art motion detectionsystems have employed integration of the Doppler signals produced over aperiod of time before an associated alarm generating circuit is enabledto provide an alarm. Although such techniques desensitize the systems toambient noise and spurious signals, while maintaining a highersensitivity for the motion detection systems, such systems can becomprised by a step and wait motion by an intruder. If the intruderwould take a full or partial step during each integration period, theintegrator circuit would not reach a level sufficient to enable thealarm generating circuit.

The time integration of the Doppler signal while for the most part beingeffective to desensitize the systems to the detection of ambient noises,inherently desensitizes the systems to the detection of an intruderwhose presence it is desired to detect. With a time integrating system,an intruder who keeps his movements slow will defeat the system sincethe integrated level of the Doppler signals will be correspondingly lowand will never exceed alarm triggering level for the alarm generatingcircuit.

SUMMARY OF THE INVENTION The present invention provides a Doppler radarintrusion alarm system having an improved noise rejection characteristicenabling the system to have a more sensitive alarm threshold settingwithout the need to employ time integration of Doppler signals.

The system includes a level detecting circuit for detecting theamplitude of Doppler signals provided as a result of noise motion andthe movements of an intruder within a protected area and a low passfilter circuit for detecting Doppler signals of frequencies that areless than a preselected cutoff frequency for the low pass filtercircuit. Doppler signals provided as a result of human motion includelow frequency components regardless of the speed at which the humantarget is moving. Accordingly, the passband of the low pass filtercircuit can be very narrow, having a cutoff frequency, for example, of5.5 Hz, with an attendant reduction in the effect of noise on thesystem.

Moreover, since only low frequency components of the Doppler signals areeffective to produce an alarm indication, the threshold setting for thelevel detecting circuit can be higher than in previously proposedsystems which rely solely on amplitude detection of the Doppler signals.

In one embodiment, the intrusion alarm system provided by the presentinvention includes Doppler radar means for producing output signalshaving Doppler frequencies indicative of a moving body within aprotected area. The signals are related in amplitude and frequency tothe characteristics of the human target.

The output signals are passed to a level detecting circuit which isresponsive to each Doppler signal in excess of a predetermined amplitudeto provide an output of a duration that is proportional to the frequencyof the received Doppler signal.

The outputs provided by the level detecting circuit are extended to alow pass filter circuit which, in a preferred embodiment, comprises adigital filter provided by the present invention, which is responsive tosignals of a duration that is greater than a preselected duration toprovide an output signal for enabling an alarm indicating means of thesystem.

The digital low pass filter circuit includes means for providing areference signal of the preselected duration and means enabled wheneverthe duration of the Doppler signal is greater than the duration of thereference signal to provide a logic level output indicative of thedetection of human motion within the protected area. The logic leveloutputs thus provided control the alarm indicating means to provide analarm indication.

The duration of the reference signal, which represents the cutofffrequency for the digital low pass filter circuit, is adjustable toenable the cutoff frequency of the digital filter circuit to be varied.1

It is pointed out that the Doppler signals do not pass through the leveldetecting and digital filter circuits and appear at the output thereofin an unaltered form. Rather, the digital filter circuit provides alogic level output whenever the frequency of a signal supplied to thedigital filter circuit is less than the cutoff frequency of the digitalfilter circuit, and a ground level output whenever the frequency of thesignal supplied to the digital filter circuit is equal to or greaterthan the cutoff frequency. Thus, the digital filter circuit has asubstantially infinite roll off characteristic. Moreover, the

bandwidth of the digital filter circuit is set at a preselected cutofffrequency and is independent of the level of the signals supplied todigital filter circuit. In addition, the use of a digital filter circuitenables information to be carried in the frequency rather than the shapeor amplitude of the waveform input to the filter circuit.

While in one application to an intrusion detection system the digitalfilter circuit is operable as a low pass filter, with certainmodifications, the digital filter circuit can provide a high passcharacteristic to thereby provide an output only in response to signalsof a frequency that is greater than a preselected frequency for thedigital filter circuit. Accordingly, the high pass digital filtercircuit includes means for providing a reference signal of a preselectedduration, representing the cutoff frequency for the high pass filter,and means enabled whenever the duration of a signal supplied to the highpass digital filter circuit is less than the duration of the referencesignal.

Moreover, the low pass digital filter circuit provided by the presentinvention can be connected in tandem with a high pass digital filtercircuit of the present invention to provide a digital bandpass or notchfilter wherein the pass band of the filter is determined by the cutofffrequencies of the low pass and high pass digital filter circuits. Thedigital bandpass filter circuit provides a logic level output only whena signal supplied to the bandpass filter circuit is within the passbandof the filter circuit as determined by the cutoff frequencies of the lowpass and high pass filter circuits which comprise the digital bandpassfilter circuit. Thus, the digital bandpass filter is a true notch filterand the pass band of the digital filter can be made as narrow asdesired.

Other features of the invention will become apparent from the followingdetailed description of the invention which makes reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a Dopplerradar motion detection system provided by the present invention;

FIG. 2 is a schematic representation of one embodiment for a signaldetecting and filter circuit for the system shown in FIG. 1 including alevel detector circuit and a low pass digital filter circuit;

FIG. 2A shows the transfer characteristic for the digital filter circuitshown in FIG. 2;

FIGS. 3 and 4 are timing diagrams showing the relationships betweensignals for the digital filter circuit shown in FIG. 2;

FIG. 5 is a schematic representation of a second embodiment for thesignal detecting and filter circuit for the system shown in FIG. 1including an active low pass filter circuit and a level detectorcircuit;

FIG. 5A shows the transfer characteristic for the active low pass filtercircuit shown in FIG. 5;

FIG. 6 is a schematic block diagram of a high pass digital filtercircuit provided by the present invention;

FIGS. 7 and 8 are timing diagrams showing the relationships betweensignals for the digital filter circuit shown in FIG. 6;

FIG. 9 is a schematic block diagram of a digital bandpass filter circuitprovided by the present invention; and

FIGS. 10-12 are timing diagrams showing the relationships betweensignals for the digital filter circuit shown in FIG. 9.

DESCRIPTION OF PREFERRED EMBODIMENTS A block diagram of a Doppler radarmotion detection system 20 provided by the present invention is shown inFIG. 1. The motion detection system 20 comprises Doppler radar apparatusincluding a transmitter 21 having an associated transmitter antenna 22,and a receiver 23 having an associated receiver antenna 24. In anexemplary embodiment, the transmitter 21 provides microwave signals at afrequency of 2.45 GI-Iz. However, it is apparent that other forms andfrequency bands of radiated energy may be used. The 2.45 GI-Iz signalsare radiated from the transmitter antenna 22 into an area to beprotected and are shown impinging on a moving body or target 30 fromwhich the radiant energy is reflected and picked up by the receiverantenna 24. In accordance with the principles of the Doppler effect,whenever there is moving body within the effective range of the Dopplerradar system, that portion of the energy reflected off the moving bodyhas its frequency shifted by the amount of the Doppler frequency. Thefrequency and the amplitude of the Doppler signal are a function of thesize, velocity and reflective characteristics of the moving body.

Digressing, it is well known that the Doppler frequency F is directlyrelated to the frequency of the transmitted energy F0, the radialvelocity Vr of the moving body, relative to the point of reception ofthe transmitted energy, and the propogation velocity Vp in accordancewith the formula:

In one application, the motion detection system 20 is used to detectunauthorized movement of an intruder within a protected area. In suchapplication wherein movements of a human target are to be detected, theDoppler frequency F varies over a rather large range. The lowest targetvelocity Vr is zero when the intruder is stopped, and the highest targetvelocity Vr one could reasonably expect from a human motion is 33 feetper second yards in 9 seconds) when an intruder is running. Thus, in themotion detection system 20 of the exemplary embodiment wherein thefrequency of the radiated energy is 2.45 GI-Iz, the frequency of Dopplersignals provided as the result of movements of a human target within theprotected area will range from 0 167 Hz for the velocity range of 0 to33 feet per second.

Although the Doppler frequency may range from 0 to 167 Hz in theexemplary system, a person walking at a normal pace will produce signalsof approximately 1 Hz to 2 Hz. Moreover, these low frequency componentsare present regardless of the velocity of the human target. The lowfrequency components are attributed to the fact that even when runningthe velocity of the intruders foot, for example, will decrease to zeroas the foot touches the ground and then increase with the next stride.correspondingly, the frequency of the Doppler signals returned to thereceiver 23 will also decrease to zero and then increase. Likewise, whenthe intruders arm swing reaches its maximum and begins to return, thefrequency of the Doppler signals resulting from reflection of theradiated energy from the arms of the intruder will also decrease to zeroand then increase.

Inasmuch as these low frequency component Doppler signals are generallyon the order of l to 2 Hz, in accordance With one embodiment, the motiondetection system has a cutoff frequency of approximately 5.5 Hz.However, if a higher source frequency were used, the system cutofffrequency could be increased correspondingly inasmuch as the range ofDoppler frequencies provided would be increased.

When signals at a frequency of 2.45 GI-Iz radiated from the transmitter21 are reflected off the moving body 30, the resultant Doppler signalsare reflected back to the antenna 24 and are coupled to the receiver 23.The output of the receiver 23 is in turn connected to a mixer circuit25. A portion of the signal energy generated by transmitter 21 iscoupled directly into the mixer 25. Thus, the signals obtained in themixer 25 include the transmitted frequency F0 and the transmittedfrequency modulated by the Doppler signals (F0 F).

The amplified Doppler signals provided at the output of amplifier 26 areextended to the input of a signal detecting and filter circuit 27 whichdetects both amplitude and frequency of the received Doppler signals andprovides a trigger signal for an alarm generating circuit 28 wheneverthe amplitude of a received Doppler signal is greater than apredetermined level and the frequency of the received Doppler signal isless than a preselected frequency.

The transmitter-21, the receiver 23, the mixer circuit 25 and theamplifier 26 may comprise a conventional low power Doppler radar system,and accordingly, the circuits for this portion of the system are notdescribed in detail. The alarm circuit 28 may comprise an indicatingdevice, an alarm relay or some other type of alarm indicator as isconventional in the art.

One embodiment for a signal detecting and filter circuit 27, shown indetail in FIG. 2, includes a level detecting circuit 35 and a low passdigital filter circuit 36 having a cutoff frequency Fc of 5.5 Hz. Aswill become apparent, the pass characteristic of the substantiallyinfinite rolloff, and accordingly, the digital filter circuit will beresponsive only to signals of frequencies less than 5.5 Hz regardless ofthe amplitude of the signals.

Referring to FIG. 2, the level detecting circuit 35, which may comprisea Schmitt trigger circuit, is responsive to received Doppler signals inexcess of a predetermined amplitude to provide an output for theduration or frequency of the Doppler signal supplied to the input of theSchmitt trigger circuit 35.

The Schmitt trigger circuit 35 includes an input switching transistorQ1, which is normally cut off, and an output switching transistor 02,which is normally saturated. The collector-emitter circuit of the inputtransistor O1 is connected across the base-emitter circuit of the outputtransistor Q2. As is understood in the art, the Schmitt trigger circuit35 has a stable state with transistor 01 normally cut off and transistorQ2 normally saturated. When the amplitude of an input signal supplied tothe base of transistor O1 is in excess of a DC voltage establishedthereon, the condition of transistors Q1 and O2 is reversed, and theoutput transistor Q2 becomes cut off while the input transistor O1 issaturated. This condition continues until the amplitude of the inputsignal on the base of input transistor 01 returns to a value below theoriginal DC bias voltage, whereupon the Schmitt trigger circuit 35reverts to its original stable state with input transistor Q1 cut offand output transistor 02 saturated. The DC bias on the base oftransistor O1 is adjustably selected by means of cut off, the Schmitttrigger circuit 35 provides a logic level output signal +V at output Awhich is of a dura-' tion which is proportional to the frequency of theDoppler signal extended to the input of the Schmitt trigger circuit 35.

The digital low pass filter circuit 36 includes a resettable monostablecircuit 40 having an associated delay circuit 41. The delay circuit 41determines the length of time for which the monostable circuit isenabled and thus, the cut off frequency for the digital filter circuit36. The delay circuit 41 is adjustable by way of an adjust network 49 tovary the length of enabling time of the monostable circuit 40. In thepresent example, the delay circuit 41 is set to provide a cutofffrequency of 5.5 Hz for the Doppler signal filter circuit 36 of themotion detection system.

The monostable circuit 40 is set to provide a +V logic level output at Bwhenever an enabling signal is extended to the set input of themonostable circuit 40 which is connected to output A of the Schmitttrigger circuit 35. The filter circuit 36 further includes an exclusiveOR gate 42 having a first input connected to the output B of themonostable circuit 40 and a second input connected to output A of theSchmitt trigger circuit 35. The exclusive OR circuit 42 compares theduration of the +V level output of the Schmitt trigger circuit 35, whichrepresents the frequency of the received Doppler signal F, to theduration of the +V level output provided by the monostable circuit 40,which represents the cutofi' frequency Fe for the digital filter circuit36.

The exclusive OR circuit 42 will be enabled to provide a +V level outputwhenever a logic level signal is present on either input, but not onboth inputs. Thus, whenever the duration of the signal. output of theSchmitt trigger circuit 35 is greater than the duration of the output ofthe monostable circuit 40, the exclusive OR circuit 42 will be enabledwhen the monostable circuit 40 times out. Alternatively, whenever theduration of the signal output of the Schmitt trigger circuit 35 is lessthan the duration of the signal output of the monostable circuit 40, theexclusive OR circuit 42 will be enabled when the output of the Schmitttrigger circuit 35 goes to zero.

The output of the exclusive OR circuit 42 is extended to an input of aNAND gate 43, a second input of which is connected to the output A ofthe Schmitt trigger circuit 35. The NAND gate 43 is enabled only whenthe exclusive OR gate is enabled while a +V level output is beingprovided by the Schmitt trigger circuit 35. Stated in another way, theNAND gate 43' will be enabled only when the duration of the signaloutput of the Schmitt trigger circuit 35 is greater than the duration ofthe output signal provided by the monostable circuit 40 or when thefrequency F of Doppler signal extended to the Schmitt trigger circuit 35is less than the cut-off frequency Fc of the digital filter circuit 36.

The output of NAND gate 43 is extended over a second NAND gate 41, whichserves as an inverter to a set input of a second resettable monostablecircuit 46. A

capacitor 45 connected to ground at the output of NAND gate 44suppresses transients.

Since the duration for which the NAND gate 43 is enabled is related tothe frequency F of the Doppler signal being detected, the secondmonostable circuit 46 is used to provide a constant duration output foreach Doppler signal extended to the filter circuit 36 which is of afrequency that is less than the cutoff frequency Fc of the digitalfilter circuit 36.

Accordingly, whenever NAND gate 43 is enabled the +V level outputprovided by NAND gate 43 and inverted by NAND gate 44 will enable themonostable circuit 46 to provide a +V level output at E for apredetermined duration.

The length of time for which the monostable circuit 46 is enabled andthus the duration of the output signal for the digital filter circuit 36is controlled by the delay circuit 48 the operation of which is adjustedvia an adjust network 50.

The low pass digital filter circuit 36 is thus responsive to each signalprovided at the output of the level detecting circuit 35 which is of afrequency that is less than the preselected cutoff frequency Fc of thedigital filter circuit to provide a +V level output. On the other hand,responsive to signals of frequencies equal to or greater than the cutofffrequency Fc, the low pass digital filter circuit 36 provides a groundlevel output. Thus, the transfer characteristic of the digital filtercircuit 36, shown in FIG. 2A has a substantially infinite roll off atthe cutoff frequency Fc, dependent of the level of the received Dopplersignal.

Examples of commercially available logic circuits suitable forapplication in the low pass digital filter circuit include the type SN74123 resettable monostable circuit for the monostable circuits 40 and46, the type SN 7486 Exclusive OR gate for exclusive OR gate 42 and thetype SN 7400 and NAND gate for gates 43 and 44.

OPERATION OF THE MOTION DETECTION SYSTEM Referring to FIG. 1, inoperation, transmitter 2] generates microwave signals at a frequency of2.45 GI-Iz, which signals are radiated via antenna 22 into the protectedarea.

If a moving body 30 is present in the effective field of the system, the2.45 Gl-Iz signals provided by transmitter 21 when reflected off themoving body 30 will be shifted in frequency, providing a Doppler signalthe amplitude and frequency of which are related to characteristics ofthe moving body 30. It will be appreciated that the amplitude of theDoppler signal obtained is a function of the strength of the reflectedsignal picked up by the receiver 23 and is thus a function of the range,size and reflection characteristics of the target. The frequency of theDoppler signal, on the other hand, is related to the velocity at whichthe target or body 30 is moving. If the moving body 30 is a humantarget, the frequency of the Doppler signals reflected back to thereceiver antenna 24 may range from O to 167 Hz for the present system.

The signals at the output of the receiver 23 which include thetransmitted frequency F and the sum and difference of the transmittedfrequency F0 and the Doppler signals (F0 i F) are mixed with thetransmitted frequency signals in the mixer circuit 25 and accordingly,the output of the mixer circuit 25 will be the Doppler frequencies. Thereceived Doppler signals are passed over amplifier 26 to the signaldetecting and filter circuit 27.

Referring to FIG. 2 and to the timing diagram given in FIG. 3, whenevera Doppler signal having a frequency less than the cutoff frequency Fefor the digital filter circuit 36 and an amplitude in excess of thethreshold setting for the Schmitt trigger circuit 35 is received at theinput of the Schmitt trigger circuit, the Schmitt trigger circuit 35 isenabled to provide an output of a duration which is proportional to thefrequency of the received signal. The output of the Schmitt triggercircuit 35, shown on line A of FIG. 3, is extended to the set input ofthe monostable circuit 40. When monostable circuit 40 is enabled, themonostable circuit provides an output, shown on line B of FIG. 3, for apredetermined duration as determined by the setting of the delay circuit41. The output of the monostable circuit 40 and the output of theSchmitt trigger circuit 35 are extended to the inputs of the exclusiveOR circuit 42.

When the outputs of the monostable circuit 40 and of the Schmitt trigger35 are +V logic levels, the exclusive OR circuit 42 remains disabled.Since in the present example the frequency of the received Dopplersignal is within the pass band of the digital filter circuit 36, theduration of the input signal provided by the Schmitt trigger circuit 35is greater than the duration of the signal output of the monostable 40as can be seen by comparing lines A and B of FIG. 3. Thus, the exclusiveOR circuit 42 will be enabled when the monostable output goes to zero orground. At such time, the exclusive OR circuit 42 will provide a +Vlogic level output as shown on line C of FIG. 3, which altogether withthe +V logic level provided to NAND gate 43 by the Schmitt triggercircuit will enable NAND gate 43 to provide a ground level output.Accordingly, responsive to the ground level output of NAND gate 43 NANDgate 44 will provide a +V level output, shown on line D of FIG. 3 toenable monostable circuit 46. When enabled, the monostable circuit 46will provide a +V level output shown on line E of FIG. 3 for a durationdetermined by the setting of the delay circuit 48.

Alternatively, referring to the timing diagram of FIG. 4 in conjunctionwith FIG. 2, whenever the received Doppler signal extended to the inputof the Schmitt trigger circuit 35 is of a level sufficient to enable theSchmitt trigger circuit but is of a frequency which is outside of thepassband of the low pass digital filter circuit 36, the output of thedigital filter circuit 36 will be at ground level.

Thus, for example, assuming the output of the Schmitt trigger circuit 35is a square wave signal of +V level and of the duration shown on line Aof FIG. 4, the output of the Schmitt trigger circuit 35 will enable themonostable circuit 40 to provide the output as shown in line B of FIG.4. However, as can be seen by comparing the wave forms of lines A and Bof FIG. 4, the received Doppler signal, which' is of a frequency greaterthan the cut-out frequency Fo, will disable the Schmitt trigger circuit35 and the input signal at A will go to ground level before themonostable circuit 40 has timed out. At such time, the exclusive ORcircuit 42 will be enabled to provide the output shown in line C of FIG.4. However, one input of NAND gate 43, which is connected to the outputof the Schmitt trigger circuit, will be at ground level, andaccordingly, the output provided by the exclusive OR circuit 42 will beineffective to enable NAND gate 43. Accordingly, the monostable circuit46 will be enabled and the output of the monostable circuit 46 willremain at ground level as indicated in line E of FIG. 4.

Second Embodiment of Signal Discriminating Circuits While the operationof the motion detection system 20 provided by the present invention hasbeen described with reference to a preferred embodiment wherein thesignal detecting and filter circuit 27 includes a digital low passfilter circuit 35, it is apparent that other types of low pass filtercircuits may be employed without departing from the scope of theinvention. For example, with reference to FIG. 5, there is shown afurther embodiment for the signal detecting and filter circuit 27 whichincludes an active low pass filter circuit 60 and a level detectingcircuit 61 connected to the output of the active filter circuit 60. Thelow pass filter circuit 60 is comprised of an operational amplifier 62and associated bias elements R3, R2 and C1, C2, the values of which areselected to provide the desired band pass characteristic for the lowpass filter circuit 60. Since the active low pass filter circuit 60 hasa finite roll-off, as shown by the transfer characteristic for thefilter circuit 60 given in FIG. a, the band width of the active filtercircuit is dependent upon the amplitudes of the signals supplied to thefilter circuit 60. Thus, a lower cutoff frequency, such as 1 Hz, isselected for the filter circuit 60 to provide an overall system bandwidth that is similar to the band width of the digital filter circuit 36shown in FIG. 2. An increased rolloff ratio can be obtained through theuse of additional stages for the low pass active filter circuit 60.

The input of the low pass active filter 60 at 63 is connected to theoutput of the amplifier 26 shown in FIG. 1 to receive the Dopplerfrequency signals provided at the output of the mixer 25. The output ofthe low pass active filter 60 at 64 is connected to the input of thelevel detecting circuit 61 which, for example, may be a Schmitt triggercircuit which is similar to the circuit 35 shown in FIG. 2.

The low pass active filter circuit 60 is operable to pass only receivedDoppler signals which are of a frequency within the passband of theactive filter circuit 60. The signals output from the active filter 60are extended to the level detecting circuit 61 which provides an outputonly when the Doppler signals passed by the filter circuit 60 are inexcess of a preselected amplitude. The output of the level detectorcircuit 61 is extended to the alarm circuit 28 (FIG. 1) which is thusenabled to provide an alarm whenever a Doppler received signal is of afrequency within the pass band of the low pass filter circuit 60 and ofan amplitude exceeding the threshold setting of the detector circuit 61.

Other Digital Filter Embodiments With certain modifications in theconnections of the digital filter 36 provided by the present inventionthe digital filter circuit can provide a high pass characteristic.

Referring to FIG. 6, there is shown a schematic block diagram of adigital high pass filter circuit 70 and an associatd wave shapingcircuit 71. The wave shaping circuit 71 is responsive to frequencysignals supplied to the input of the wave shaping circuit 71 to providelogic level pulses at output F which are related in duration to thefrequency of the signals input to the wave shaping circuit 71.

The high pass digital filter circuit 70 includes a monostable circuit 72having an input connected to the output F of the wave shaping circuit 71and an output G connected to an input of an exclusive OR gate 73. Asecond input of the exclusive OR gate 73 is connected to the output F ofthe wave shaping circuit 71.

The monostable circuit 72v is set by each signal provided at the outputF of the wave shaping circuit 71 and remains enabled to provide anoutput at G for a duration which is determined by a timing circuit 74associated with the monostable circuit 71. As will become apparent, thesetting of the timing circuit 74 determines the cutoff frequency for thehigh pass filter circuit and is adjustable by way of an adjust network78. The output H of the exclusive OR gate 73 is connected to an input ofan AND gate 75. The OR gate 73 has a second input connected to theoutput F of the wave shaping circuit 71. The AND gate provides an outputat I whenever signals input to wave shaping circuit 71 are within thepass band of the filter circuit 70. The AND gate 75 may comprise a pairof NAND gates connected in tandem in manner of NAND gates 43 and 44shown in FIG. 2.

In operation, an input signal having a frequency greater than the cutofffrequency of the high pass filter circuit 70 will enable monostablecircuit 72 to provide an output G, which output is of a durationdetermined by the timing circuit 74 associated with monostable circuit72. If the duration of the input signal (line F, FIG. 7) is less thanthe duration of the monostable output (line G, FIG. 7), the exclusive ORcircuit 74 will be en'- abled when the input signal goes to groundlevel, providing the +V level output shownon line H of FIG. Since +Vlevel outputs are provided at points G and H, the AND gate 75 will beenabled to provide the level output, shown online I of FIG. 7, which isrelated to the frequency of the input signal supplied to the waveshaping circuit 71. Since the monostable 72 is operable for a knownperiod of time, the duration of the output signal will be proportionalor related to the frequency of the received signal.

Alternatively, if the frequency of the input signal is equal to or lessthan the cutoff frequency of the high pass digital filter, the signaloutput of the wave shaping circuit 71, shown on line F of FIG. 8, willbe of a greater duration than the output of the monostable, shown online G of FIG. 8. Accordingly, when the monostable times out, theexclusive OR gate 73 will be enabled by the output F of the wave-shapingcircuit 71, but NOR gate 75 will not be enabled since the inputconnected to the output of the monostable is at ground level.

Referring to FIG. 9, there is shown a schematic block diagram of adigital bandpass filter cicuit and an associated wave shaping circuit81. The digital bandpass filter circuit 80 includes a low pass section82, including a monostable circuit 83, a timing circuit 84 and ad justnetwork 87 associated with the monostable'circuit 83 for determining thecutoff frequency for the low pass section 82, an exclusive OR gate andan AND gate 86, and a high pass section 92 connected in tandem with thelow pass section 82, including a monostable circuit 88, a timing circuit89 and adjust network 90 associated with the monostable circuit 88 fordetermining the cutoff frequency for the high pass section 87, anexclusive OR gate 91 and an AND gate 93. A transient suppressingcapacitor 94 is connected to ground at the output of AND gate 86.

The upper cutoff frequency for the bandpass filter 80 is determined bythe cutoff frequency for the low pass section 82 and the lower cutofffrequency for the bandpass filter 80 is determined by the high passsection 92. Thus, the cutoff frequency for the low pass section 82 ishigher than the cutoff frequency for the high pass section 92. Thebandpass filter circuit 80 will pass only signals of frequencies thatare less than the cutoff frequency of the low pass section 82 andgreater than the cutoff frequency of the high pass section 87.

In operation, when a signal of a frequency that lies within the passband of the digital bandpass filter circuit 80 is supplied to thewave-shaping circuit 81, the wave-shaping circuit 81 provides an output,shown in line J of FIG. 10, of a duration which is proportional to thefrequency of the received signal. The output of the wave-shaping circuit81 enables the monostable 83 of the low pass filter section 82 toprovide an output shown in line K of FIG. 10, the duration of which isdependent upon the setting of the timing circuit 84 associated withmonostable circuit 83. Since the received signal, shown on line J ofFIG. 10, is lower in frequency than the cutoff frequency for the lowpass section as can be seen by comparing the durations of the waveformsshown on lines J and K of FIG. 10, the monostable circuit 83 will timeout before the output of the wave-shaping circuit 81 goes to ground. Atsuch time exclusive OR gate 85 will be enabled by the output of thewave-shaping circuit 81 to provide the output shown on line L of FIG.10. In addition, AND gate 86 will be enabled by the output of theexclusive OR gate 85 and the output of the wave-shaping circuit 81 toprovide the output shown on line M of FIG. 10. The output of the ANDgate 86 is connected to the set input of the monostable 88 of the highpass section 92. Accordingly, whenever an output is provided by AND gate86, the monostable 88 of the high pass section 92 will be enabled toprovide the output shown on line N of FIG. 10. The duration of theoutput provided by monostable 88 is determined by the setting of theassociated timing circuit 89. When the output provided by the AND gate86 of the low pass section 82 returns to a ground level, exclusive ORgate 91 of the high pass section 92 will be enabled to provide theoutput shown in line P of FIG. 10. In addition, AND gate 93 of the highpass circuit section 92 will also be enabled to provide the output shownon line Q of FIG. 10.

Alternatively, assuming the frequency of the signal received by thewave-shaping circuit 81 is greater than the high frequency cutoff forthe bandpass filter 80, the monostable 83 of the low pass section 82will be enabled by the output signal shown on line J of FIG. 11,provided by the wave-shaping circuit 81 to provide the output shown online K of FIG. 11. Since the received signal is greater in frequencythan the cutoff frequency of the bandpass filter 80, the duration of thesignal provided by the wave-shaping circuit 81 is less than the durationof the output provided by the monostable circuit 83. Accordingly, whenthe input signal J returns to ground level, the monostable circuit 83remains enabled. Therefore, when the exclusive OR gate 85 is enabled bythe output provided by the monostable circuit 83, the AND gate 86 of thelow pass section 82 will not be enabled, and moreover, the high passsection 92 of the bandpass filter circuit 80 will not be enabled.Consequently, the output of the bandpass filter circuit 80 will remainat ground level.

In the event that the frequency of the signal received by thewave-shaping circuit 81 is less than the lower cutoff frequency of thebandpass filter circuit 80, the monostable circuit 83 will be enabled bythe output of the wave-shaping circuit 81, shown in FIG. J of FIG. 12,to provide the output shown in line K of FIG. 12.

When the monostable 83 times out, the exclusive OR gate 85 will beenabled to provide the output shown in Line L of FIG. 12 and inaddition, the AND gate 86 will be enabled by the output of exclusive ORgate 85 and the output of the wave-shaping circuit 81 to provide theoutput shown in line M of FIG. 12.

When the AND gate 86 of the low pass section 82 is enabled, the outputof the AND gate 86 will enable the monostable circuit 88 of the highpass section 92 to provide the output shown in line N of FIG. 12. Sincethe frequency of the received signal is lower than the cutoff frequencyfor the high pass section 92, the duration of a signal supplied to themonostable 88 from the AND gate 86 will be greater than the duration ofthe output provided by the monostable circuit 88. Accordingly, when themonostable output goes to ground level, exclusive OR gate 91 will beenabled to provide the output shown on line P of FIG. 12. However, ANDgate 93 will not be enabled since the output of the monostable 88 is at0 level. Therefore, the bandpass filter will provide a ground leveloutput.

We claim:

1. In an intrusion alarm detection system for detecting the presence ofa human intruder moving within a protected area, means for producingDoppler signals within a predetermined frequency range whenever there isa moving body within the effective radiated energy field of the system,said Doppler signals being related in amplitude and frequency tocharacteristics of the moving body, and signal detecting means fordetecting the amplitude and frequency of each of said Doppler signals,said signal detecting means being re sponsive to a single cycle of aDoppler signal having an amplitude greater than a preselected level anda frequency less than a predetermined frequency within saidpredetermined frequency range which is indicative of human motion toprovide an output signal indicative of the detection of a human intrudermoving within the protected area for each cycle of the Doppler signalhaving an amplitude greater than said preselected level and a frequencyless than said preselected frequency.

2. A system as set forth in claim 1 wherein said signal detecting meansincludes level detecting means respon sive to each cycle of a Dopplersignal in excess of a predetermined amplitude to provide a logic leveloutput of a duration representative of the frequency of the Dopplersignal, and digital filter means responsive to each logic level outputof said level detecting means that is in excess of a predeterminedduration to provide a further logic level output which indicates thedetection of a human intruder moving within the protected area.

3. A system as set forth in claim 1 wherein said signal detecting meansincludes low pass active filter means for passing only Doppler signalswhich are of a frequency less than a predetermined cutoff frequency forsaid active filter means and a level detecting circuit responsive toeach cycle of the Doppler signals passed by said active filter meanswhich are of an amplitude greater than a predetermined level forproviding an output signal indicative of the detection of a humanintruder moving within the protected area.

4. In an intrusion detection system for detecting the presence of ahuman intruder moving within a protected area, means for producingDoppler frequency signals within a predetermined frequency rangewhenever there is a moving body within the effective radiated energyfield of the system, said Doppler signals being related in amplitude andfrequency to characteristics of the moving body, and signal detectingmeans including means for discriminating between Doppler signals havingan amplitude in excess of a given level and Doppler signals having anamplitude less than said level and means for discriminating betweenDoppler signals having a frequency less than a preselected frequencywithin said predetermined range which is indicative of human motion andDoppler signals having a frequency equal to or greater than saidpreselected frequency, said signal detecting means being responsive to asingle cycle of a Doppler signal having a frequency that is less thansaid preselected frequency and having an amplitude in excess of saidgiven level to provide an output signal indicative of the detection of ahuman intruder moving within the protected area for each cycle of theDoppler signal having an amplitude greater than said given level and afrequency less than said preselected frequency.

5. In an intrusion alarm detection system for detecting the presence ofa human intruder moving within a protected area, a Doppler radar systemfor producing Doppler signals within a predetermined frequency rangewhenever there is a moving body within the effective radiated energyfield of the system, said Doppler signals being related in amplitude andfrequency to characteristics of said moving body, level detecting meansresponsive to each Doppler signal in excess of a given level to providea first output signal of a duration representative of the frequency ofthe Doppler signal for each cycle of the Doppler signal, and digitalfilter means responsive to each output signal provided by said leveldetecting means that is of a duration greater than a predeterminedduration which is indicative of human motion to provide a second outputsignal indicative of the detection of a human intruder moving within theprotected area.

6. A system as set forth in claim 5 wherein said digital filter meansincludes reference means for providing a reference signal of apredetermined duration representing a cutoff frequency for said digitalfilter means, and output means for comparing the reference signal witheach output signal provided by said level detecting circuit to providesaid second output signal whenever the duration of the signal providedby said level detecting means is greater than the duration of saidreference signal.

7. A system as set forth in claim 6 wherein said output means includesfirst means responsive to said reference signal and said first outputsignal to provide an enabling signal whenever the duration of said firstoutput signal is greater than the duration of said reference signal andsecond means controlled by said enabling signal and said first outputsignal to provide said second output signal whenever the frequency ofthe first output signal is less than the cutoff frequency of the digitalfilter means.

8. A system as set forth in claim 7 wherein said reference meansincludes means for adjusting the cutoff frequency of said digital filtermeans.

9. A system as set forth in claim 6 wherein said output means includesfor providing a second output of a predetermined duration for eachDoppler signal detected.

10. In an intrusion alarm detection system for detecting the presence ofa human intruder moving within a protected area, a Doppler radar systemfor producing Doppler signals within a predetermined frequency rangewhenever there is a moving body within the ef-' fective radiated energyfield of the system, said Doppler signals being related in amplitude andfrequency to characteristics of said moving body, level detecting meansresponsive to each Doppler signal in excess of a given level to providea first logic level output signal of a duration representative of thefrequency of the Doppler signal for each cycle of the Doppler signal anddigital filter means including monostable circuit means responsive toeach output signal provided by said level detecting means to provide alogic level reference signal of a predetermined duration representing acutoff frequency for said digital filter means, and gating means forcomparing the reference signal with each output signal provided by saidlevel detecting means to provide a second logic level output signalindicative of the detection of a human intruder moving within theprotected area whenever the duration of the output signal provided bysaid level detecting means is greater, than the duration of saidreference signal.

11. A system as set forth in claim 10 wherein said gating means includesexclusive OR means controlled by said reference signal and said firstoutput signal to provide an enabling output signal whenever the durationof said first output signal is greater than the duration of saidreference signal, and output gating means controlled by said enablingsignal and said first output signal to provide said second logic leveloutput.

12. A system as set forth in claim 10 wherein said monostable circuitmeans includes an adjustable timing network for enabling the duration ofthe reference signal to be varied.

13. A system as set forth in claim 1 1 wherein said digital filter meansincludes further monostable circuit means responsive to said secondlogic level output signal to provide an output signal of a predeterminedduration.

14. A system as set forth in claim 13 wherein said further monostablecircuit means includes an adjustable timing network for enabling theduration of the output signal provided by said further monostablecircuit means to be varied.

"UNIT-E STATES PATEM 0mm CERTIFECATE J CUIRRECTIGN Patent No. 3,803,599Dated April 9 3.97

Inventofls) Mi hae'i B. McLean and Allen Rasmussen It is crtified thaterrbr appears I in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

fialumn 14,- "iineh after "includes" insert means a Signed arid sealedthis 10th day of Septemb'r 197M.

{371E111} A-ctest:

i IcCOY M. GIBSON; JR; v c. MARSHALL DAM mttesting Officer Commissionerof Patents PC1-1050 'UNiT-ED STATES PATEM OFFER (a .69) v r v m I m uCELR'IH iCAi E OF CUKRECTIQN Patent NO. 3,803,599 Dated April 9 1974Invencofls) Miehaei B MCLafi and Alln Rasmussen It is certified thaterror appears in the above-identified patent end that said LettersPatent are hereby corrected as shown below:

in z *5 Column 34, 'iineyg after ineludes" insert means 0 Signed andsealed this 10th day of September 19m.

Actest:

moo?! M. GIBSON, J-R'. c. MARSHALL DANN.

btesting Officer Commissioner of Patents

1. In an intrusion alarm detection system for detecting the presence of a human intruder moving within a protected area, means for producing Doppler signals within a predetermined frequency range whenever there is a moving body within the effective radiated energy field of the system, said Doppler signals being related in amplitude and frequency to characteristics of the moving body, and signal detecting means for detecting the amplitude and frequency of each of said Doppler signals, said signal detecting means being responsive to a single cycle of a Doppler signal having an amplitude greater than a preselected level and a frequency less than a predetermined frequency within said predetermined frequency range which is indicative of human motion to provide an output signal indicative of the detection of a human intruder moving within the protected area for each cycle of the Doppler signal having an amplitude greater than said preselected level and a frequency less than said preselected frequency.
 2. A system as set forth in claim 1 wherein said signal detecting means includes level detecting means responsive to each cycle of a Doppler signal in excess of a predetermined amplitude to provide a logic level output of a duration representative of the frequency of the Doppler signal, and digital filter means responsive to each logic level output of said level detecting means that is in excess of a predetermined duration to provide a further logic level output which indicates the detection of a human intruder moving within the protected area.
 3. A system as set forth in claim 1 wherein said signal detecting means includes low pass active filter means for passing only Doppler signals which are of a frequency less than a predetermined cutoff frequency for said active filter means and a level detecting circuit responsive to each cycle of the Doppler signals passed by said active filter means which are of an amplitude greater than a predetermined level for providing an output signal indicative of the detection of a human intruder moving within the protected area.
 4. In an intrusion detection system for detecting the presence of a human intruder moving within a protected area, means for producing Doppler frequency signals within a predetermined frequency range whenever there is a moving body within the effective radiated energy field of the system, said Doppler signals being related in amplitude and frequency to characteristics of the moving body, and signal detecting means including means for discriminating between Doppler signals having an amplitude in excess of a given level and Doppler signals having an amplitude less than said level and means for discriminating between Doppler signals having a frequency less than a preselected frequency within said predetermined range which is indicative of human motion and Doppler signals having a frequency equal to or greater than said preselected frequency, said signal detecting means being responsive to a single cycle of a Doppler signal having a frequency that is less than said preselected frequency and having an amplitude in excess of said given level to provide an output signal indicative of the detection of a human intruder moving within the protected area for each cycle of the Doppler signal having an amplitude greater than said given level and a frequency less than said preselected frequency.
 5. In an intrusion alarm detection system for detecting the presence of a human intruder moving within a protected area, a Doppler radar system for producing Doppler signals within a predetermined frequency range whenever there is a moving body within the effective radiated energy field of the system, said Doppler signals being related in amplitude and frequency to characteristics of said moving body, level detecting means responsive to each Doppler signal in excess of a given level to provide a first output signal of a duration representative of the frequency of the Doppler signal for each cycle of the Doppler signal, and digital filter means responsive to each output signal provided by said level detecting means that is of a duration greater than a predetermined duration which is indicative of human motion to provide a second output signal indicative of the detection of a human intruder moving within the protected area.
 6. A system as set forth in claim 5 wherein said digital filter means includes reference means for providing a reference signal of a predetermined duration representing a cutoff frequency for said digital filter means, and output means for comparing the reference signal with each output signal provided by said level detecting circuit to provide said second output signal whenever the duration of the signal provided by said level detecting means is greater than the duration of said reference signal.
 7. A system as set forth in claim 6 wherein said output means includes first means responsive to said reference signal and said first output signal to provide an enabling signal whenever the duration of said first output signal is greater than the duration of said reference signal and second means controlled by said enabling signal and said first output signal to provide said second output signal whenever the frequency of the first output signal is less than the cutoff frequency of the digital filter means.
 8. A system as set forth in claim 7 wherein said reference means includes means for adjusting the cutoff frequency of said digital filter means.
 9. A system as set forth in claim 6 wherein said output means includes means for providing a second output of a predetermined duration for each Doppler signal detected.
 10. In an intrusion alarm detection system for detecting the presence of a human intruder moving within a protected area, a Doppler radar system for producing Doppler signals within a predetermined frequency range whenever there is a moving body within the effective radiated energy field of the system, said Doppler signals being related in amplitude and frequency to characteristics of said moving body, level detecting means responsive to each Doppler signal in excess of a given level to provide a first logic level output signal of a duration representative of the frequency of the Doppler signal for each cycle of the Doppler signal and digital filter means including monostable circuit means responsive to each output signal provided by said level detecting means to provide a logic level refereNce signal of a predetermined duration representing a cutoff frequency for said digital filter means, and gating means for comparing the reference signal with each output signal provided by said level detecting means to provide a second logic level output signal indicative of the detection of a human intruder moving within the protected area whenever the duration of the output signal provided by said level detecting means is greater than the duration of said reference signal.
 11. A system as set forth in claim 10 wherein said gating means includes exclusive OR means controlled by said reference signal and said first output signal to provide an enabling output signal whenever the duration of said first output signal is greater than the duration of said reference signal, and output gating means controlled by said enabling signal and said first output signal to provide said second logic level output.
 12. A system as set forth in claim 10 wherein said monostable circuit means includes an adjustable timing network for enabling the duration of the reference signal to be varied.
 13. A system as set forth in claim 11 wherein said digital filter means includes further monostable circuit means responsive to said second logic level output signal to provide an output signal of a predetermined duration.
 14. A system as set forth in claim 13 wherein said further monostable circuit means includes an adjustable timing network for enabling the duration of the output signal provided by said further monostable circuit means to be varied. 